1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly relates to a semiconductor device that periodically updates a DLL (Delay Locked Loop) circuit.
2. Description of Related Art
Because the present invention relates to a DLL circuit and a refresh circuit, a refresh operation and a DLL operation are briefly explained first as the background techniques of this invention to facilitate understanding thereof. The refresh operation is periodically performed to hold data stored in memory cells in a DRAM (Dynamic Random Access Memory). The refresh operation includes an auto-refresh operation performed by external instructions and a self-refresh operation autonomously performed particularly when an operation of a semiconductor device is stopped (in a power down state). The time interval of the self-refresh operation is specified by a periodic signal generated by an oscillator in the semiconductor device. While the periodic signal generated by the oscillator has a flexibility depending on its design, its time interval can be, for example, 7.6 μs according to the interval of the auto-refresh operation.
The DLL operation is for adjusting the phase of internal clock signals so that the semiconductor device can output read data in synchronization with external clocks. It is known that the DLL operation has relatively large power consumption and is not always performed during an operation of the semiconductor device, but is performed at a certain time after the refresh operation ends, as disclosed in Japanese Patent Application Laid-open No. 2004-273106, for example. Hereinafter, such a periodic activation of the DLL circuit is called “periodic update of the DLL circuit”.
Japanese Patent Application Laid-open No. 2004-273106 discloses a technique related to a refresh operation of a semiconductor device including a DLL circuit.
Such a circuit is desired that controls a DLL operation and a refresh operation that are periodically performed, respectively, without having any problems and at a reduced area.